Characteristics of active power devices, which are important components in a switching power supply, are critical to the performance of the switching power supply. With the development of semiconductor technologies, conversion efficiency of circuits composed of active power devices such as Power Factor Correction (PFC) circuits or Direct Current (DC) to DC (D2D) conversion circuits is up to 97% at present, and the power density of these circuits has reached a considerably high level. However, the characteristics of Si-material-based active power devices have approached to a theory limit and there is relatively small space left for further development, thereby further improvement of efficiency and power density of switching power supplies is impeded.
Active power devices based on wide band gap materials such as gallium nitride (GaN) or silicon carbide (SiC) have relatively small internal resistance and small switching loss, thereby are capable of bearing relatively high operation temperature, and thus the efficiency and power density of the switching power supplies can be further improved.
Active power devices based on wide band gap materials usually include three terminals, one terminal among which is a control terminal, i.e., a gate electrode, configured to control on and off of the devices. Taking Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) as an example, the three terminals are: a gate electrode, a source electrode and a drain electrode. Generally, the active power devices can be classified into two types: a normally-on type and a normally-off type. Taking a normally-on type MOSFET device as an example, when the voltage between the gate electrode and the source electrode is zero, the device is turned on; and when the voltage between the gate electrode and the source electrode is a negative voltage, the device is turned off. Taking a normally-off type MOSFET device as an example, when the voltage between the gate electrode and the source electrode is a positive voltage, the device is turned on; and when the voltage between the gate electrode and the source electrode is zero, the device is turned off. However, the problem when normally-on type switching devices are used is circuit start-up problem. This problem will be explained taking a Buck circuit as an example.
FIG. 1 is a schematic diagram for illustrating a Buck circuit composed of normally-on type switching devices. As shown in FIG. 1, switching elements Q1 and Q2 in the Buck circuit are normally-on type semiconductor devices, for example, normally-on type GaN devices. The circuit is desired to output a voltage from a middle point O to a capacitor Co via an inductor L1. However, in the initial state of the circuit, i.e., at the time when no power is applied on the circuit, the DC input voltage Vin is zero, and an auxiliary power supply Vaux and a control/driving module C&D do not provide control signals to Q1 and Q2, and thus the voltages between the gate electrodes and the sources electrodes of Q1 and Q2 are zero, i.e., Q1 and Q2 are in an on state. When the circuit is powered up, i.e., Vin is set up and is not equal to zero, since the timing when the Vaux and C&D set up control signals is later than the timing when Vin is set up, i.e., the voltages between the gate electrodes and the source electrodes of Q1 and Q2 have not yet reached a negative voltage to keep Q1 and Q2 being in an off state, and thus the currents in Q1 and Q2 will flow from a positive terminal “+” of Vin directly to the ground G and thereby the circuit may be damaged. That is to say, a problem of start-up exists in a circuit composed of normally-on type switching devices.
In view of FIG. 1, FIG. 2 shows a schematic diagram illustrating how to solve the start-up problem existing in a circuit composed of normally-on type switching devices by using an electronic switch. As shown in FIG. 2, in the loop formed by the Vin, Q1 and Q2, an electronic switch Qin is connected in series. Qin is a normally-off type switch, for example, a Metal Oxide Semiconductor (MOS) device based on Si. The voltage at the gate electrode of the normally-off type switching device is zero before the power is applied, i.e., the switching device is in an off state. When Vin is applied to the circuit, during the time period before the C&D finishes setting up of control signals, Qin is in an off state. Thus, the phenomenon that the currents in Q1 and Q2 directly flow into the ground can be avoided, and thereby the safety of the circuit may be guaranteed. When the Vaux and the C&D finish setting up of the control signals, i.e., when the driving signals of Q1 and Q2 start to work normally, Qin is kept being in an on state. In this way, a safe start-up of the circuit is realized. However, one drawback of such solution is that the voltage stress of Qin is the same as that of Q1 and Q2, both of which are Vin. Qin is generally a MOS device based on Si, and in a case when Qin is under the same voltage level as semiconductor devices based on wide band gap material such as MOS devices based on GaN, the loss caused by the on-resistance of Qin is not negligible. Further, such an electronic switch does not serve as a switching device that performs power conversion in a power conversion circuit, but generally serves as an auxiliary electronic switch when the power conversion circuit adopts normally-on type devices. Generally, the electronic switch has a relatively low operation frequency, usually lower than 1 kHz.
In order to solve the problem of the withstand voltage of the additional Si-based device and to make the GaN-based device capable of replacing the conventional Si-based MOS device without changes in control and driving schemes, a solution in FIG. 3 is proposed. FIG. 3 is a schematic diagram in which a combination of serially connected normally-off type switching device and normally-on type switching device is used to simply replace normally-off type switching devices in the circuit. As shown in FIG. 3, a Si-based normally-off type switching device QL having a withstand voltage of 40V and a GaN-based normally-on type switching device QH having a withstand voltage of 600V are connected in series to form a combination. A source electrode of QH is connected with a drain electrode of QL, and a gate electrode of QH is connected with a source electrode of QL. The drain electrode D of QH serves as the drain electrode of this combination, and the source electrode S of QL serves as the source electrode of this combination. The combination may have normally-off control characteristics similar to conventional Si-based device, and the advantages of the GaN device are also utilized. However, the solution as shown in FIG. 3 increases driving loss, loop inductance, electromagnetic interference and reverse recovery loss. Further, parameter matching of devices is hard to be fulfilled, and thereby the good characteristics of the GaN-based devices cannot be exerted thoroughly.